How would i go about storing Humanoid properties?

Your code works for me (make sure API Services are enabled in your game settings). Literally, all I did was change userId to UserId. Even then, I feel like the code should have still worked, because I believe userId is deprecated, but still functional.

local datastore = game:GetService("DataStoreService"):GetDataStore("NPS")

game.Players.PlayerAdded:Connect(function(player)
    player.CharacterAdded:Connect(function(character)
        local humanoid = character.Humanoid

        local key = "user-" .. player.UserId

        local storeditems = datastore:GetAsync(key)

        if storeditems then
            humanoid.MaxHealth = storeditems[1]
            humanoid.Health = storeditems[2]    
        else
            local items = {humanoid.MaxHealth, humanoid.Health}
            datastore:SetAsync(key, items)
        end
    end)
    player.CharacterRemoving:Connect(function(character)
        local items = {character.Humanoid.MaxHealth, character.Humanoid.Health}
        local key = "user-" .. player.UserId

        datastore:SetAsync(key, items)
    end)
end)

Also, like Davydas said, you'll want to consider using an event other than CharacterAdded and CharacterRemoving. When I tested this, I immediately received a warning, regarding the overflow of requests we were sending to the datastore. Imagine how bad this could be if you had a full server of this! Try thinking of when you really need to store this information, such as before the player leaves the game.